basically, there’s a padawan to the sith lord. (not the same idea but still).

There’s always another waiting to take over when the #1 is overthrown. Only two are Sith, but the way of the Sith basically is that if you’re the head guy you can expect to be tossed out at some point by your #2 who has his #3 waiting to kill him, etc etc. Vader was just pretty shitty about finding a successor.

Bingo! This precedent was seen at least twice in the films: Once when Count Dooku tries to entice Obi-Wan into joining him to overthrow the Emperor, and once when Vader does the same thing with Luke. The Force Unleashed is the first revelation that perhaps Luke wasn’t the first person Vader tried to find to help him overthrow the Emperor. (Of course, we know this “new” apprentice doesn’t succeed, which begs the question: “what happens him?” Either he ends up being killed, or perhaps he switches to the Light side of the Force at some point?)